Silicon carbide single crystal, which has become of interest as a semiconductor material, is generally produced through a sublimation process employing silicon carbide powder serving as a raw material. In the sublimation process, the silicon carbide powder and a seed crystal substrate are placed in the interior of a graphite-made growth crucible such that the powder and the substrate face each other, and the silicon carbide raw material is heated to 1,800 to 2,400° C. in an inert gas atmosphere. The thus generated silicon carbide sublimate gas reaches the seed crystal substrate, which is maintained at a temperature suitable for crystal growth, whereby a silicon carbide single crystal is deposited on the substrate.
In the sublimation process, sublimate gas components, e.g., Si, Si2C, SiC2 and SiC, are generated from the silicon carbide raw material, and some of the gas components are deposited onto the seed crystal substrate to thereby grow a silicon carbide single crystal. In the sublimation process, the silicon carbide single crystal is grown through sublimation and deposition of a solid compound. Therefore, the sublimation process involves the problems in 1) that the growth rate of the single crystal is low and 2) that when the growth rate is increased, generation of crystal defects and polycrystallization tend to occur.
During the growth process of the silicon carbide single crystal, the composition of the sublimate gas varies depending on various factors including sublimation and decomposition of the silicon carbide powder serving as a raw material, interaction between the sublimate gas components in a vapor phase and reaction of the gas components with graphite constituting the inner wall of the growth crucible. When the silicon carbide raw material is heated, the silicon component, which has a high vapor pressure, is readily gasified, but the carbon component tends to remain in the raw material. Therefore, during the heating process, sublimation of the silicon component contained in the silicon carbide raw material precedes sublimation of the carbon component. This phenomenon is one of the factors that cause variation in the composition of the sublimate gas. Other factors causing variation in the composition of the sublimate gas include the sublimation temperature of the raw material, the composition of the raw material and time-course change in the temperature profile in the growth crucible.
Variation in the composition of the sublimate gas at a region in the vicinity of the surface of the seed crystal substrate on which the single crystal is grown is considered to cause generation of crystal defects in silicon carbide crystals and formation of polytypes or growth of anisotropic crystals (i.e., polycrystallization), which lead to deterioration of crystallinity of the single crystal. Therefore, production of a silicon carbide single crystal of high quality requires control of the aforementioned factors that cause variation in the composition of the sublimate gas.
Conventionally, several measures have been taken in order to improve crystallinity of a silicon carbide single crystal. For example, when a silicon carbide single crystal is grown, the growth rate thereof is lowered to thereby suppress generation of crystal defects, or the time for growth continuity of the single crystal is shortened so as to reduce variation in the composition of a sublimate gas. However, these measures are not satisfactory in terms of the quality and stability of a silicon carbide single crystal produced through the sublimation process.
In view of the foregoing, the following improvement in the sublimation process has been proposed in JP-A HEI 6-316499, for example. The improvement relates to a method for reducing variation in the composition of a sublimate gas, which method comprises disposing a silicon raw material and a carbon raw material separately, reacting gas components generated from the silicon raw material with the carbon raw material to thereby form silicon carbide, and sublimating the thus formed silicon carbide to produce a silicon carbide single crystal. However, this method inevitably involves a fundamental problem associated with the sublimation process, i.e., variation in the composition of the sublimate gas with the progress of sublimation of the silicon carbide. In addition, in this method, production of a silicon carbide single crystal requires a two-step process, leading to relatively long production time.
In addition, JP-B SHO 51-8400 discloses a method that comprises evaporating silicon under heating in a reaction crucible, reacting the resultant silicon vapor with carbon vapor obtained through evaporation of carbon contained in the inner wall of the crucible, and transferring the resultant reaction gas into a silicon carbide deposition chamber to thereby deposit a silicon carbide single crystal onto the inner wall of the chamber. However, this method entails a drawback in that the growth rate of the silicon carbide single crystal is low since the vapor pressure of carbon is lower than that of silicon.
As described above, there has not yet been established an effective method for controlling factors causing variation in the composition of a sublimate gas to thereby grow a silicon carbide single crystal of high crystallinity.
In view of the foregoing, an object of the present invention is to provide a method for growing, through the sublimation process, a large-sized silicon carbide single crystal with few crystal defects on a seed crystal substrate at a high growth rate and in a reliable manner, with an atmosphere gas controlled to reduce variation in the composition of a sublimate gas and provide an apparatus for carrying out the method.
Another object of the invention is to provide a silicon carbide single crystal, with crystal defects reduced and with quality and stability maintained sufficiently high.